1. Field of the Invention
This invention relates to a tube protection device and method for use in a nuclear steam generator and, more particularly to a device which protects defect regions of such a tube from being damaged or distorted during removal of the tube from the generator for subsequent defect analysis and testing.
2. State of the Prior Art
Tube-type heat exchangers, commonly referred to as steam generators, as employed in nuclear reactor power plants are well known in the art. U.S. Pat. No. 4,079,701 to Hickman et al. and U.S. Pat. No. 4,276,856 to Dent et al., assigned to Westinghouse Electric Corporation, the common assignee of this invention, is an example of such a steam generator. The development of defects in the heat exchanger tubes is also a well-known problem. The need to plug a heat exchange tube arises when a defect permitting a leak in such a tube develops, creating both an ineffective heat exchanger problem and, more seriously, a radioactive contamination problem. Particularly, a defect in a heat exchanger tube permits primary fluid, which recirculates through the tube and a nuclear power generator, to leak into the secondary fluid which surrounds the tubes; since the former is generally radioactive, the secondary fluid surrounding the tubes becomes comtaminated. U.S. Pat. No. 4,369,662, assigned to the common assignee herein, discloses a plug installation apparatus which is capable of quickly installing a plug in such a heat exchanger tube to render it inoperable, and of verifying proper installation of the plug.
Because of the seriousness of the problems presented by defects developing in steam generator tubes, it is also the practice in the art to remove tubes having such defects from the generator, to permit conducting defect analyses and testing, including destructive examination. Prior art techniques for removal of such tubes include placing a tapered, threaded mandrel into the end of the tube containing the defect, and then removing the tube using a hydraulic puller assembly which pulls on the mandrel and the tube. The force required to remove the tube, however, frequently imposes excessive loads on the tube causing the latter to elongate; since the defect region of the tube is typically the weakest part of the tube, the tube is prone to distortion and breakage in the defect region, thus destroying the defect region and precluding the intended examination and analyses. Previously, solutions to avoiding the problem of destroying the defect region during the pulling of the tube have been proposed wherein a device both pulls and protects the tube simultaneously; such prior art devices, however, have generally been limited to use with only a limited extent of the tube adjacent an open end thereof (e.g., the bottom three feet) and thus are incapable of protecting against destruction or distortion of defects in the tube located at positions remote from the open end.